I have no idea which plug is right for this engine. To test the right heat range, run it on both plugs. The one that gives the highest RPM is the right plug. The speed at peak is a combination of the fuel mixture and ignition timing. If a plug is too hot then you will have to richen the mixture to quench the plug and a smokey exhaust results, the synthetic oil will not burn but will vaporize to a finer smaller droplets. If the plug is about right then there is not as much quenching of the plug and peak will occur with a leaner mixture and at a higher RPM.